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  1. Abstract CeOs 4 Sb 12 , a member of the skutterudite family, has an unusual semimetallic low-temperature L -phase that inhabits a wedge-like area of the field H —temperature T phase diagram. We have conducted measurements of electrical transport and megahertz conductivity on CeOs 4 Sb 12 single crystals under pressures of up to 3 GPa and in high magnetic fields of up to 41 T to investigate the influence of pressure on the different H – T phase boundaries. While the high-temperature valence transition between the metallic H -phase and the L -phase is shifted to higher T by pressures ofmore »the order of 1 GPa, we observed only a marginal suppression of the S -phase that is found below 1 K for pressures of up to 1.91 GPa. High-field quantum oscillations have been observed for pressures up to 3.0 GPa and the Fermi surface of the high-field side of the H -phase is found to show a surprising decrease in size with increasing pressure, implying a change in electronic structure rather than a mere contraction of lattice parameters. We evaluate the field-dependence of the effective masses for different pressures and also reflect on the sample dependence of some of the properties of CeOs 4 Sb 12 which appears to be limited to the low-field region.« less
    Free, publicly-accessible full text available April 1, 2023
  2. Abstract

    High entropy alloys (HEA) are an unusual class of materials where mixtures of elements are stochastically arrayed on a simple crystalline lattice. These systems exhibit remarkable functionality, often along several distinct axes: e.g., the examples [TaNb]1-x(TiZrHf)xare high strength and damage resistant refractory metals that also exhibit superconductivity with large upper critical fields. Here we report the discovery of anf-electron containing HEA, [TaNb]0.31(TiUHf)0.69, which is the first to include an actinide ion. Similar to the Zr-analogue, this material crystallizes in a body-centered cubic lattice with the lattice constanta = 3.41(1) Å and exhibits phonon mediated superconductivity with a transition temperaturesTc ≈ 3.2 K and uppermore »critical fieldsHc2 ≈ 6.4 T. These results expand this class of materials to include actinide elements, shows that superconductivity is robust in this sub-group, and opens the path towards leveraging HEAs as functional waste forms for a variety of radioisotopes.

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